The invention relates to an apparatus for measuring the level of a liquid in an enclosure. It is more particularly applicable to measuring the level of the liquid water contained in the pressurizer of a pressurized water nuclear reactor (PWR reactor).
In PWR reactors, a high pressure is maintained in the primary circuit by a pressurizer constituted by a sealed enclosure in which the pressure of the primary circuit is maintained by equilibrium between the liquid phase and the gaseous phase of the water.
Numerous apparatuses are known, which make it possible to measure the liquid phase level in the pressurizer, e.g. measuring devices using the pressure difference. However, these known apparatuses cause problems of representativeness (hysteresis, variations, readjustments). Moreover, the transition between the liquid phase and the vapour phase is not always clearly defined. In certain cases, there is an intermediate two-phase area, with vapour bubbles in the liquid water. A pressure difference device does not make it possible to detect such a situation. It is for this reason that level measuring apparatuses based on the absorption variations of a radiation, when said radiation passes through media of different natures, have been the subject of considerable interest. Thus, apparatuses for measuring the water level in a pressurizer by means of one or more radiation sources disposed in the vicinity of the enclosure wall and one or more radiation detectors receiving the radiation having traversed the enclosure are known. The measurement of the signal received is then used for deducing the level of the surface of the liquid and/or the local density of the two-phase mixture.
For example, French Patent Application No. 79 22218, filed on September, 1979, in the name of the Commissariat a l'Energie Atomique and entitled: "Process and apparatus for measuring the level of a liquid in an enclosure" describes an apparatus of this type, in which the result of the measurements is compared with a library of preestablished curves and relating to the different possible two-phase configurations.
However, this apparatus has the disadvantage of not permitting a direct and rapid measurement, because it requires the comparison of axial measurement maps with a library of preestablished curves. Moreover, it requires a high radiation source of approximately 1000 Ci, which causes radiation protection problems.
Finally, for distinguishing the radiation from the source from that of the activated water and that of the active deposits appearing on the walls, it is necessary to have a supplementary detector, positioned in such a way that it is not exposed to the radiation from the sources, which makes it possible to correct in the total counting rate, that part which is due to the activity of the water and the deposits.